Voltage source inverters are increasingly utilized in many applications, such as, for example, motor drive systems for electric vehicles or hybrid-electric vehicles within automotive applications. In these applications, a voltage source inverter (VSI) may supply an alternating current (AC) to a dependent device, such as, for example, an AC motor.
Thermal management issues concerning power switches within the voltage source inverter (VSI) may arise when the VSI operates at low or zero output frequency; that is, when the VSI is applying zero or low voltage across the phases while receiving a constant direct current (DC) link voltage, referred to as input voltage. When operating at low or zero output frequency, the average voltage produced by the VSI is low in comparison to the DC link voltage, while the output current might be the switch rated current.
Thermal management issues can arise at zero or low output frequency due to slowly changing AC current. That is, under these conditions, a power switch in the VSI might be required to carry maximum rated current continuously or for a very long period, causing relatively constant high power dissipation inside the power switch. This condition represents a worst-case operating condition for a switch in the VSI. A conventional solution is to time-limit the VSI operation at zero or low output frequency and full output current. Unfortunately, this conventional solution can be problematic for electric and hybrid vehicle operation because it generally limits the time during which maximum torque can be applied to the electric motor during vehicle start. Another conventional solution is to use pulse width modulation with a switching frequency where the phase with the highest phase current is not switched, but is held at the rail voltage for a period of time to reduce losses in switching. This conventional solution increases noise (called “current ripple”) in the output voltage. A higher switching frequency may be used to reduce current ripple, but the higher switching frequency adds to switching losses in the phases that are switched.
In view of the foregoing, it would be desirable to provide a system that would overcome these and other disadvantages and improve existing voltage source inverters and operation of voltage source inverters. Accordingly, it is desirable to increase VSI current capability at low output frequencies. In addition, it is desirable to provide a suitable current ripple. Furthermore, other desirable features and characteristics will become apparent from the summary and subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background.